The present invention relates generally to digital systems for controlling analog processes and more specifically to an improved hybrid system of a digital-analog control of an analog process.
Analog processes, for example weighing systems, as exemplified by U.S. Pat. No. 4,111,336 to Ward et al. include a control system having a digital command signal converted by a digital to analog converter to an analog signal to be used in the control system with an analog weight signal to control the process. The analog weight signal is also converted to a digital signal by an analog to digital converter to be used to create the original digital command signal. These systems generally include a closed loop including a digital to analog converter to create a command valve and an analog to digital converter to provide a measurement signal back to the digital portion of the system. These systems must connect with the errors produced by the lack of tracking of the digital to analog converter and the analog to digital converter. They will both have independent drift and independent gain temperature coefficients. Thus, there is inherent error in the control and measurement loop.
Even the more sophisticated systems using a microprocessor as illustrated in U.S. Pat. No. 4,054,784 to Ricciardi still provide a digital to analog converter at the output of the microprocessor to provide analog control signals for the motor and uses an analog to digital converter in the feedback loop to provide measured or sensed conditions of the process back to the microprocessor. The problems produced by the lack of tracking of the digital to analog converter and the analog to digital converter is the same as that for the previously mentioned Ward et al. patent. The Ricciardi patent also includes the microprocessor continuously in the measurement and control loop. This preoccupies the microprocessor and is an inefficient use of its time.
Thus, there exists a need for a hybrid control system which takes full advantage of digital and analog systems while minimizing component errors.